Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various paper or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
US 2005001419 (DIGIMARK) discloses a colour laser engraving method and a security document including an opaque surface layer and one or more coloured sub-layers. A laser provides openings in the surface layer to expose the colour of the sub-layer thereby creating colour images and text.
WO 2009/140083 (3M) discloses methods to generate a colour image in a multilayer article containing at least one thermally activatable layer coated from a composition including a non-linear light to heat converter, a leuco dye, a thermal acid generator and a solvent. A colour image is formed in the colour forming layer upon activation with non-linear light beam radiation (300-1500 nm).
U.S. Pat. No. 7,158,145 (ORGA SYSTEMS) discloses a three-wavelength system (440, 532 and 660 nm) for applying coloured information to a document by means of wavelength-selective bleaching of chromophoric particles in a layer close to the surface.
U.S. Pat. No. 4,720,449 (POLAROID) discloses a thermal imaging method for producing colour images on a support carrying at least one layer of a colourless compound, such as di- or triarylmethane, by direct application of heat or by conversion of electromagnetic radiation into heat. The laser beam may have different wavelengths, typically in a range above 700 nm with at least about 60 nm apart so that each imaging layer may be exposed separately to convert a colourless triarylmethane compound into a coloured form, such as yellow, magenta, cyan or black, by controlling the focusing depth of the laser beam source to each colour forming layer. The colour forming compositions include di- or triarylmethane compounds, infrared absorbers, acidic substances and binders.
U.S. Pat. No. 4,663,518 (POLAROID) discloses a laser printing method for activating heat sensitive image forming dyes in three different layers on a support to provide an identification card containing a coloured pictorial image of the card holder, coloured text and machine readable digital code.
Laser marking can be categorized into the “additive” imaging techniques, comparable to e.g. inkjet, while laser engraving is a “subtractive” imaging technique. Often in falsifying security cards, the addition of information has been proven to be easier than the subtraction of information. For example, it is possible to completely change a photograph of the card holder by adding more hair, a moustache and glasses. Many approaches have been developed to prevent such falsification.
One approach involves a change of the information content on the security document. For example, WO 2008/084315 (AXALTO) discloses a secure identification document comprising a first set of identification data and a second set of identification data obtained by duplicating the first set of identification data, which takes the form of a reverse image of the first set of identification data. A disadvantage of including an image and its reverse image is the reduction of space available for other type of information.
Another approach involves the addition of security features such as, for example, a watermark as U.S. Pat. No. 7,097,899 (AGFA) or the use of a lenticular lens in US 2003183695 (DIGIMARC). Generally, the addition of such security features represents an increase in the cost of manufacturing security documents. It often also requires installing at the customer site more costly and sophisticated equipment, e.g. including a coating device, needed to assemble the security documents.
Since methods for falsification of security documents continue to develop and improve, it remains a constant battle to protect security documents against falsification and counterfeiting. Therefore a need exists to provide simple and cost-effective methods for securing documents.